A New Geometrical Design of Octagonal Fractal Microstrip Patch Suitable Formobile & Wi-Max Applications

Transcription

1 A New Geometrical Design of Octagonal Fractal Microstrip Patch Suitable Formobile & Wi-Max Applications 1 P VenuMadhav, 2 Dr. M. Sivaganga Prasad 1,2 Dept. of ECE, PVPSIT, AP, India 2 Dept. of ECE, CR Reddy College of Engineering, AP, India Abstract This paper presents design of an octagonal parasitic micro strip patch antenna with an operating frequency ranging from 1.5 GHz to 5GHz that can be used for wi-max as well as mobile application. The maximum bandwidth can be achieved by controlling the distance between the patch antenna and by adjusting the probe feed position. The proposed antenna should have a larger impedance bandwidth. Study of current view on the patch is investigated it is observed that optimization is to be performed on antennaphysical size. Keywords Parasitic Patch, Wi-max, Microstrip Antenna, Dielectric Constant, Resonant Frequency, VSWR I. Introduction Microstrip antennas are used where thickness and conformability [1] to the host surfaces are the key requirements. The primary limitation of this type of antenna is the bandwidth [2], which is less than 5% for most single substrate designs.some of the principle advantages of micro strip patch antennas are lightweight, low profile, conformal, linear and circular polarization possibility and easy to implement feed position, however in array applications a microstrip line feed may often be more appropriate. The impedance bandwidth of traditional microstrip antenna is only a few per cent, therefore it becomes important to develop broadband technique to increase the bandwidth of the microstrip antenna.since patch antennas can be directly printed on to a circuit board these are becoming increasingly popular with in mobile phone market. The microstrip antenna was first proposed by GA. Desschamps in In its most basic form, a microstrip patch antenna consists of a radiation patch on one side of a dielectric substrate which has a ground plane on the other side. The ground plane is normally modestly lager than the active patch. The current flow is along the direction of the feed wire, so the vector potential and thus the electric field E follows the current. Such a simple patch antenna radiates a linearly polarized wave. A patch antenna is fabricated by etching the antenna element pattern in metal trace bonded to an insulating dielectric substrate with a continuous metal layer bonded to the opposite side of the substrate which forms a ground plane. The key drawback of this type of antennas is narrow bandwidth. Inorder to support wider bandwidth thick substrates are to be used, but thick substrates provide tightly bound surface wave modes which represent a loss mechanism in the antenna. The patch antennas may be of a square, rectangular, circular, triangular, elliptical shapes or any other continuous shape. The two most common geometries rectangular and circular are widely in use. Square patches are used to generate pencil beam and rectangular patch for a fan beam. Circular patches can also be used but the calculation of current distribution is relatively complex. The size of a microstrip antenna is inversely proportional to its frequency of operation. The most common board is a dual copper coated polytetrafluoroethylene (Teflon) fiber glass as it allows the microstrip antenna to be curved to conform to the shape of the mounting surface. The patch is generally made of conducting material lie copper or gold. The radiating patch and the feed lines are photo etched on dielectric substrate. Microstrip antennas also suffer limitations like low bandwidth, low efficiency and low gain and low power handling capacities. Some of the limitations can be overcome by the use of thick substrates, cutting slots in metallic patch, introducing parasitic patches etc. This paper proposes a new dimensional antenna with octagonal shape and whose application can be seen in wi-max or mobile applications. The octagonal patch shape is chosen as it has the advantage of occupying less metalized area on substrate than other existing configurations. II. Design of Octagonal Microstrip Patch In this paper the antenna is designed to support 1.5 GHz 5GHz frequencies, so as to meet the demand of mobile or wi-max application. The frequency can be changed by varying the antenna size and geometrical modification. A coaxial probe feed [3] coplanar octagonal patch antenna is to be designed for a resonant frequency below 3 GHz, on a finite ground plane. Appropriate feeding technique has to be used since the feed can be placed at any place on the patch to match with its input impedance (50Ω). The thickness of the metallic strip is restricted to t<<λ 0. The strip and ground plane are separated by a dielectric sheet referred to as the substrate. The dielectric constants are usually in the range of 2.2 <Ԑr<12. The proposed antenna consists of ground plane that is slightly larger than that of the radiating patch and is fed by a 50Ω microstrip line. The dielectric material selected is FR4 substrate which has a dielectric constant of 4.4 and the height of the substrate is 1.6mil. The fig. 1 shows the coplanar octagonal patch whose dimensions (length) are obtained by using the special designed antenna modelling tool. Fig. 1: Proposed Octagonal Patch Antenna The length of each side of an inner octagon is 1cm. the area of each octagon is 4.83cm 2. There are altogether 14 octagons and the 66 International Journal of Electronics & Communication Technology

2 ISSN : (Online) ISSN : (Print) overall area of the entire patch is 67.62cm. The shape of the patch is also designed in the form of an octagon whose effective side is equal to 4cm and hence the S0 (effective area) of the patch is 77.25cm. But since the inner octagons are removed in the etching processes thus forming via slots in the form of triangles and square that are generated because of the cascade octagon patches. The side of each triangle is 0.5 cm. Area of triangle of side 0.5cm =1/2 *b*h =0.125cm 2 No of such triangles are 27 Hence total electrical length that will be removed in the form of slots for triangle shape is 3.375cm 2.(S 1 ) Similarly, area of square of side 0.5cm=s*s=0.25cm 2 No of square shapes in the patch = 3 Hence the electrical length removed in terms of square shape is 0.75cm 2.(S 2 ) The overall area that is removed due to etching of the patch is S=S 0 -(S 1 +S 2 ) = =73.125cm 2 Where S effective electrical area of the patch. A. Calculation of Resonant Frequency The octagonal patch has a side length of a and printed on a substrate of thickness h with a relative dielectric constant Ԑr. The frequency of operation of the patch antenna is generally determined by the length L. the critical or center frequency fc can be given as The resonant frequency is given as C- Velocity of light (1) Ԑr- thickness of the substrate. IJECT Vo l. 5, Is s u e Sp l - 3, Ja n - Ma r c h 2014 B. Feeding Technique Varieties of methods are available for feeding the microstrip patch. The main classifications are contact type and non-contacting method in contacting method RF power is fed directly to the radiating patch using a connecting element such as a microstrip line, coaxial feed or probe feed, whereas aperture coupled feed and proximity coupled feed [3] come under non contacting feed mechanisms. The coaxial feed or probe feed is a very common technique used for feeding into microstrip patch antennas (Pues and Van de Capelle,1984). The inner conductor of the coaxial connector extends through the dielectric and is soldered to the radiating patch, while the outer conductor is connected to the ground plane. The main advantage of this type of feeding scheme is that the feed can be place at any desired location inside the patch in order to match with its input impedance (50Ω). This feed method is easy to fabricate and has low spurious radiation. Generally, a high dielectric material is used for bottom substrate and a thick, low dielectric constant material is used for the top substrate to optimize radiation from the patch. A technique in which the two dielectric substrates are used such that the feed line is between the two substrates and the radiating patch is on top of the upper substrate is called as electromagnetic coupling scheme or proximity coupled feed technique. The main advantage of this feed technique is that it eliminates spurious feed radiation and provides very high bandwidth (up to 13%) due to overall increase in the thickness of the microstrip patch antenna. The table below demonstrates the type of substrates that offer higher bandwidths. Table 1: Substrate parameters Parameters Substrates Bakelite Fr4 glass epoxy RO4003 Taconic TLC RT Duroid Resonant frequency 10 GHz 10 GHz 10 GHz 10 GHz 10 GHz side 9.15 mm mm mm mm mm Frequency 8 GHz 8 GHz 8 GHz 8 GHz 8 GHz Return Loss db db db db db VSWR Gain 3 dbi 4 dbi 5 dbi 5.5 dbi 6.5 dbi Directivity 6.5 dbi 6.5 dbi 7 dbi 7 dbi 7.5 dbi Bandwidth % % 7.59 % 8.80 % 15 % Antenna efficiency % 50 % 67.5 % 70 % 80 % Radiating efficiency % 50 % 70 % 67.5 % 80 C. Input Impedance A variety of approximate models have been proposed for calculation of input impedance for a probe fed patch. These include the transmission line model, cavity model and spectral domain model. These models work well for thin substrates, giving reliable results for h/λ 0 < The cavity model has the advantage modelling the patch cavity as a parallel RLC circuit, while the probe inductance is modelled as a series inductor. The input impedance of this circuit is approximately described by (2) From the equation if the feed is located at x=x f and 0 y f S, the input resistance at resonance for the dominant TM 10 mode can be expressed as Kara has suggested an expression for x f that does not need calculation of radiation resistance. It is given as D. Selection of the Substrate The selection of a substrate material [6] is a balance between the required electrical, mechanical and environmental performance required by a design vs constraints. A dielectric material with a lowest tangent (tan δ) is preferred. The loss tangent is a metric International Journal of Electronics & Communication Technology 67

3 of the quantity of electrical energy which is converted to heat by a dielectric. The lowest possible loss tangent maximizes the antenna efficiency. The use of substrates with higher dielectric constants also tightens fabrication tolerances. The tolerance of the dielectric value is also of significant importance in manufacturing yield. A Monte Carlo type analysis using the cavity model is a good method of estimating antenna manufacturing when an etching tolerance, substrate thickness tolerance, feed point location tolerance and dielectric tolerances are known. Substrate electrical and physical parameters also vary with temperature. Recent work done by kabacik and Bailkowski[4] indicate that Teflon / fiber glass substrates can have significant variation of dielectric constant for many airborne and space borne applications. The performance variations are due to changes in material dielectric properties- thermal expansion had a minor effect on microstrip antenna performance [7]. FR4 is an inexpensive and can be used for many commercial applications[8] where the operational frequency is below 2 GHz. All substrates and laminates have different requirements for processing. Details of fabrication issues and methods may be found in the literature and directly from manufacturers. Other fabrication options such as screen printing conductive inks directly on substrates have also been investigated. III. Design Procedure for Octagonal Patch The three essential parameters for the design of octagonal patch are Frequency of operation (f0) (1.5GHz 5GHz) Dielectric constant of the substrate (Ԑr) FR4 substrate as it offers better results and is cost effective Height of the substrate (h) 1.65mm which is optimal for having maximum radiation and has less leaky waves. And provides a balance between conductor and dielectric loss. IV. Simulation Software The structure of the proposed antenna is designed using sonnet lite which is a free version with limitation on the memory size up to 1MB. It has in built designer editor and supports with various antenna substrate materials and ability to measure the current, far field and responses. It also supports 2D and 3D views. The fig. below represents the antenna model and various stages during its design phase. The shaded triangles represent the vias that will be removed during the etching process thus leading to reduction in the electrical length. The below fig. contains the actual octagonal fractal patches that are used for radiating the signal that is coupled through the feed point which is identified as port 1 in the figure. The combination of square and triangle vias makes the antenna a fractal hybrid model. Fig. 3: Octagonal Patches With Metal Surface A. Dimension of the Ground Plane The dimension of the ground plane must be chosen enoughlarge to replace entirely the infinite ground plane. Inpractice these dimensions are in order of some wavelengths. Finite ground plane gives rise to diffraction of radiation fromthe edges of the ground plane resulting in changes in radiationpattern, radiation conductance, and resonance frequency. The experimental investigations on the resonantfrequency of aoctagonalfractal hybrid patch as a function of the size, current view and geometry are as follows B. Geometry Project Units Box Size Number of Dielectrics Metals Used Via Metals Used Top of box Bottom of box Number of Polygons Mils 160 by 140 mils (2.65 mils high) 2 (1 metalization levels) Copper: Cnd:5.8e7 T:1.2 CR:0 Lossless: Cnd:INF Lossless: Cnd:INF Lossless Lossless 32 all staircase) C. Dielectric Layers Thickness (mils) Material Name Erel Dielectric Loss Tan Diel Cond Mrel 1 Copper FR Magnetic Loss Tan Fig. 2: Patch Antenna Representing Vias in Triangles and Squares 68 International Journal of Electronics & Communication Technology

4 ISSN : (Online) ISSN : (Print) IJECT Vo l. 5, Is s u e Sp l - 3, Ja n - Ma r c h 2014 D. Parameter Values Freq (GHz) Eeff Z Resistance Ω Capacitance Fig. 5: 3D-View of Current Levels on the Surface of the Patch Fig. 4: 2D- View of Current Levels on the Surface During First Run With Port at the Bottom of the Patch From the above fig. it is observed that the current distribution is not uniformly distributed over the surface and some glitches are also found where the current is also most negligible. So the port is moved to the center of the patch which is as indicated in the fig. below. Fig. 4: 2D-View of Current Levels on the Surface of the Patch V. Conclusion An octagonal fractal hybrid antenna model is proposed which works in the frequency range of 1GHz 5GHz whose resonant frequency can be fine-tuned by varying the feed point location. The cascaded octagonal patch structures thus resemble fractal structure and the via slots created in the form of squares and triangles reduce the electrical length by a few centimetres. During the first run the port was placed near the edge of the patch and found that the current distribution on the surface of the patch is unequal. Later the port is moved to the middle of the patch and measurements have demonstrated the potential of the patch to radiate current with equal distribution over the surface. Another fact that influenced the selection of the change in feed port is substrate material as electrically thin substrate with large permittivity is more suitable. Hence, further investigations are to be made with a clear understanding that the thickness by varying the materials used as substrate and the metal on the surface, thickness of the substrate and other parameters are to be studied. The results may vary after the patch antenna model is fabricated and tested in real time environments. References [1] AbolfazlAzari, A new super wideband fractal microstrip antenna, IEEE transactions on Antennas and Propagation, Vol. 59, No. 5, May 2011 [2] "MerihPalandoken, Berlin institute of technology", Germany, chapter 3, Microstrip Antennas, [Online] Available: [3] D. M. Pozar, D. H. Schaubert, eds.,"microstrip Antennas", IEEE Press, 1995 (a collection of significant manuscripts on microstrip antennas). [4] R. A. Sainati,"CAD of Microstrip Antennas for Wireless Applications", Artech, 1996 (comes with some CAD freeware). [5] P. Bhartia, K. V. S. Rao, R. S. Tomar,"Millimeter-wave Microstrip and Printed Circuit Antennas", Artech, [6] J.-F. Zürcher, F. E. Gardiol,"Broadband Patch Antennas", Artech, [7] K. C. Gupta et al.,"microstrip Lines and Slotlines", 2nd ed., Artech, [8] Binu Paul, S. Mridula, C. K. Aanandan, P. Mohanan, A new microstrip patch antenna for mobile communications and Bluetooth applications. International Journal of Electronics & Communication Technology 69

5 DR. M. Siva Ganga Prasadwas born in Vijayawada, A.P., India on April 22,1974. He received his bachelor s degree in engineering from M V S R Engineering College and M.Tech in VLSI from Bharat Institute of Higher Education & Research, Chennai. He received his Ph.D degree in Electronics and Communication Engineering from JNTU, Kakinada in the area of Wireless communications and Antennas. He has published papers in various National and International Journals, Conferences and is currently working as a Professor & HOD,Dept of ECE, in Sir CR Reddy college of Engg, Eluru. Mr. P VenuMadhav was born in Vijayawada, AP. India on 5th October Received his MSc in Electronics from NagarjunaUniversity, and M.Tech in Radar and Microwave Engineering fromuniversity College of Engineering, Andhra University (Campus)& currently pursuing PhD from KL University. His main research interests are antenna design and characterizationfor wireless applications. In particular, his current R&D focuses on smalland broadband antennas/arrays for mobile systems.currently working as Assistant Professor in ECE Department at P.V.P Siddhartha institute of technology, Vijayawada. 70 International Journal of Electronics & Communication Technology